dt-bindings: arm: move sprd board file to vendor directory

We've created a vendor directory for sprd, so move the board bindings to
there.

Link: https://lore.kernel.org/r/20200110063755.19804-2-zhang.lyra@gmail.com


Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Olof Johansson <olof@lixom.net>